The government owns rights in the present invention pursuant to grant number AI31431 from the National Institute of Allergy and Infectious Diseases.
Human ehrlichial infections are increasingly recognized in the United States and worldwide. Ehrlichiae are small, pleomorphic, obligately intracellular bacteria which are members of the family Rickettsiaceae (Chen et al., 1994). Hematopoietic cells are the primary targets of ehrlichial infection. Ehrlichiae that cause human disease include the mononuclear phagocyte pathogens Ehrlichia sennetsu and E. chaffeensis and a granulocytic ehrlichia closely related to E. phagocytophila and E. equi (Anderson et al., 1991; Chen et al., 1994; Rikihisa, 1991). Human monocytic ehrlichiosis in the United States appears to be caused by E. chaffeensis (Anderson et al., 1991; Everett et al., 1994). Human monocytic ehrlichiosis was first reported in the United States in 1987 (Fishbein et al, 1987, Maeda et al, 1987), and the isolation of the causative agent, Ehrlichia chaffeensis, from a patient was reported in 1991 (Dawson et al, 1991). The disease has been documented serologically in 30 states of the United States, in Africa, and in Europe (Fishbein et al, 1994, Morais et al, 1991, Uhaa et al, 1992). Human monocytic ehrlichiosis is a moderate to severe illness, even life-threatening in some cases (Fichtenbaum et al, 1993, Paddock et al, 1993, Tal and Shannahan, 1995). Ticks are the most likely vector. Most patients have a history of tick bite or exposure to ticks prior to onset of illness (Fishbein et al, 1994). E. chaffeensis-specific DNA sequences have been amplified from ticks (Anderson et al, 1993).
The immunodominant antigens of various ehrlichial species are cross-reactive, making diagnosis of a particular species more difficult. For example, Ehrlichia chaffeensis is genetically and antigenically closely related to E. canis and E. ewingii, canine pathogens, and E. muris, a Japanese rodent isolate (Anderson et al, 1992, van Vliet et al, 1992, Wen et al, 1995). One study has shown that rabbit and human E. chaffeensis antisera react with more than 20 E. chaffteensis antigens ranging from 20 to 200 kDa (Chen et al., 1994). The 120-, 66-, 58-, 44-, 28-, and kDa proteins are the immunodominant antigens of E. chaffeensis which react with serum antibodies from persons who have recovered from human monocytic ehrlichiosis (Chen et al, 1994, Dumler et al, 1995). The 22 kDa antigen cross-reacts with E. canis. The 66, 64, 55 and 44 kDa proteins cross-react with E. sennetsu, and the 55 and 44 kDa antigens cross-react with E. risticii, and the major immunodominant antigens, 66, 55 and 44 kDa, cross-reacted with E. chaffeensis, E. canis, E. sennetsu and E. risticii (Chen et al., 1994). Others have demonstrated serologic cross-reactions among E. equi, E. phagocytophila and human granulocytic ehrlichia (Dumler et al., 1995) and cross-reactions among Neorickettsia helminthoeca and E. risticii, E. sennetsu and E. canis (Rikisha, 1991).
A general method for identifying a rickettsial or related organism, including the various Ehrlichial species, is based on the amplification of the 16S rRNA gene. However, this test does not distinguish between species or even strains, some of which are more pathogenic than others. Unfortunately, the immunodominant proteins and their encoding genes, that are specific for each species and that could be used for diagnosis and for the development of vaccines or treatments based on surface antigenicity have not been isolated.
The present invention seeks to overcome this and other deficiencies in the art by providing the first example of an isolated gene that encodes an immunodominant antigen in the human pathogen, Ehrlichia chaffeensis. This discovery enables the development of diagnostic techniques, and the production of specific antigens and antibodies to be used in active and passive immunization techniques for preventative and therapeutic applications for both animal and human subjects.